An electric double-layer capacitor of such a construction as shown for example in FIGS. 18 and 19 has been proposed as a capacitor of a large capacity. As shown in FIG. 18, in this electric double-layer capacitor, a capacitor element 31 is contained in a metallic casing 36 with a closed bottom, and after a drive electrolyte is impregnated into this capacitor element 31, the capacitor element 31 is put into the bottom-closed metallic casing 36, and an opening portion thereof is sealed by an opening-sealing member 37. The outside of the metallic casing 36 is covered with a synthetic resin-made sleeve 38.
As shown in FIG. 19, in the capacitor element 31, sheet-like polarizable electrodes 32a and 33a, each formed by mixing and kneading, for example, activated carbon, carbon and polytetrafluoroethylene (PTFE) as a binder together, are bonded by an electrically-conductive adhesive respectively to foil-like, sheet-like or mesh-like metal current collectors 32b and 33b to which lead-out leads have already been fixed, thereby forming electrode members 32 and 33, and the pair of electrode members are wound into a roll via separators 34 and 35.
The current collectors 32b and 33b are made larger in width than the sheet-like polarizable electrodes 32a and 33a to provide protruding lead portions 32c and 33c, respectively, and during the winding operation, these protruding lead portions 32c and 33c are tilted toward the center of the capacitor element 31 (by swaging) to provide lead surfaces for surface-to-surface contact purposes.
With respect to upper and lower surfaces of the capacitor element 31 formed by swaging, the lower surface 32c is held in contact with an inner surface of a bottom surface portion 36a of the metallic casing 36, and the upper surface 33c is held in contact with an inner surface of an electrically-conductive terminal plate 39 extending through the opening-sealing member 37.
With respect to a method of sealing the opening-sealing member 37, a transverse drawn groove for retaining the opening-sealing member 37 is formed at the metallic casing 36, and after the opening-sealing member 37 is put on this transverse drawn groove, the open end portion of the metallic casing 36 is curled inwardly, thereby fixing the opening-sealing member 37. In order to further enhance the sealing effect, the curled open end portion of the metallic casing 36 bites into an annular rubber portion 40 provided at the opening-sealing member 37.
As a prior art literature relating to the invention of the present Application, there is known, for example, JP-A-10-275751.
However, in the above conventional large-capacity capacitor, the upper surface 33c of the capacitor element 31 serves as an anode lead surface while the lower surface 32c thereof serves as a cathode lead surface, and the lower surface 32c is electrically connected to the inner bottom surface 36a of the metallic casing 36, and therefore the metallic casing 36 is a cathode.
When this capacitor is used in a high-temperature and high-humidity environment, there is encountered a problem that the drive electrolyte leaks along the inner side surface of the metallic casing 36 to the exterior since the open end portion of the metallic casing 36, serving as the cathode, is disposed in biting engagement with the annular rubber portion 40 provided at the opening-sealing member 37.
With respect to this leakage, the metallic casing 36 is the cathode, and therefore hydroxide ions are produced at its sealed opening portion as a result of electrochemical reaction of the water content of the drive electrolyte, and the electrolyte exhibits a higher alkalinity due to the action of the hydroxide ions and positive ions of the electrolyte. The drive electrolyte, exhibiting the alkaline nature, moves along the inner side surface of the metallic casing 36 to deteriorate the rubber portion 40 held in contact with the open end portion, thereby lowering the sealing performance.
On the other hand, there are occasions when a plurality of capacitors are connected in series in order to enhance voltage-withstanding properties. In this case, a metallic casing of the first capacitor is plus, and the metallic casings of the second to last capacitors are alternately plus and minus.
When this metallic casing is plus, a current-collecting plate is minus. Here, also, when for example, tetraethylammonium fluoroborate is used as a solute of the drive electrolyte, tetrafluoroborate anions which are negative ions in the drive electrolyte approach the sealed opening portion, and a reaction, represented by (Chemical Formula 1), occurs, and then hydronium ions are produced by a reaction represented by (Chemical Formula 2), so that the drive electrolyte exhibits an acid nature.BF4−+H2O BF3(OH)−+HF  (Chemical Formula 1)HF+H2O→H3O++F−  (Chemical Formula 2)
The acid component of this drive electrolyte moves along the inner side surface of the metallic casing, and deteriorates the rubber portion held in contact with the open end portion, so that the sealing performance is lowered.
When the drive electrolyte thus leaks to the exterior, this has invited not only a problem that the lifetime of the capacitor is shortened but also a problem that when the drive electrolyte deposits in a bridging manner on positive and negative portions of a wiring pattern on a printed circuit board, a malfunction of the circuit is incurred since the drive electrolyte, leaking to the exterior, has ion-conductive properties.
This invention seeks to solve the above conventional problems, and an object of the invention is to provide a capacitor of a large capacity in which a drive electrolyte will not leak to the exterior even when the capacitor is used for a long period of time in a high-temperature and high-humidity environment, and also to provide a method of connecting the capacitors.